New Polymer Could End Battery Fires, Quadruple Efficiency

Photo: Toyota UK

Lithium-ion batteries are at the heart of the phones, tablets, cars, and planes we use every day, but they have an inconvenient habit of occasionally bursting into flames. Now, a team of scientists have found they can replace the unstable, fire-prone chemicals in batteries with an exceptionally stable polymer.

Researchers at UNC Chapel Hill, led by chemist Joseph DeSimone, were originally looking for a material that would keep marine life from adhering to the hulls of ships. But like most great discoveries, it led down a different path.

While testing the material, the team realized this perfluoropolyether, or PFPE, could dissolve lithium salt, an indicator needed to produce conductivity in batteries. “Most polymers don’t mix with salt, but this one did,” says grad student and head researcher Dominica Wong. “And it was nonflammable.”

Lithium-ion batteries create power by moving ions from the negative electrode to the positive electrode. When it re-charges, the ions go the opposite direction. The plain lithium batteries like the AAs in your remote control can only discharge. To recharge, li-ion batteries require an electrolyte, typically an ion-rich liquid like dimethyl carbonate (DMC), which is flammable even at room temperature. “They’re cousins to gasoline,” DeSimone says.

DeSimone and his team have been working with PFPE for years, and during their research, the crew found that another polymer electrolyte, polyethylen glycol or PEG, and PFPE could combine to dissolve salt, and potentially function as an electrolyte. When his team attached the PFPE to dimethyl carbonate, an electrolyte traditionally used in batteries, the resulting PFPE-DMC was a polymer that could move a battery’s ions with insane levels of efficiency while remaining stable.

How does that compare to batteries now? The battery in a Tesla or Prius, using a regular electrolyte, has a transference rate of around 0.2, which works, but is far from ideal. The PFPE electrolyte measured around 0.91, almost approaching “unity” — a transference of 100 percent.

“The holy grail in batteries is a lithium-air battery, which has the power density equivalent to a fuel tank,” DeSimone says. “Everyone’s been working on it, but one of the linchpins is that [regular] electrolytes aren’t compatible with oxygen.” With the application of the new PFPE electrolyte, this type of power-dense battery might actually be possible.

While the lithium-ion batteries in a smartphone are small and used for a comparatively short ownership period, blowing up is rarely an issue. But a larger setup like those found in the Boeing Dreamliner or Tesla Model S, the big reserves of electrolyte fluids are more sensitive. The team’s testing shows that a PFPE electrolyte can remain stable down to -194 degrees Fahrenheit (-90 degrees Celsius) and up to 392 degrees Fahrenheit (200 degrees Celsius). An electrolyte that doesn’t catch fire or freeze could blow open doors for aeronautical, automotive, and marine applications. It would also mean a battery that wouldn’t freeze in a Minnesota winter.

Commercial appearances of this electrolyte are — as with most radical discoveries — still years away, but DeSimone and his team are continuing their research.

“We’re stoked,” DeSimone said. “You’ve got nonflammability, this transference number, and the ability to do it at high temperatures.” And that’s a trifecta that could be the next great battery revolution we need.

The Boeing 787 has suffered from battery fires that grounded the fleet last year. Photo: NTSB